Complete annotation of all functional sequences in the human genome remains a major challenge a decade after its initial sequencing. This pertains in particular to gene regulatory elements, many of which are located far away from their target genes, but play fundamental roles in human biology. Significant progress towards annotation of the gene regulatory architecture has been made in recent years predominantly using cultured human cells. However, large-scale studies in mice, as well as anecdotal examples identified in human studies, have indicated the existence of large populations of gene regulatory sequences with very restricted temporal and tissue-specific activity during mammalian development. Despite their critical importance in human development and disease, this set of regulatory sequences will likely be missed by approaches restricted to cell lines or adult tissues. To fill this gap, the major objective for this U54 application is to generate catalogs of developmentally active gene regulatory sequences using existing high throughput data production pipelines for genomic approaches including ChlP-Seq, MethylC-Seq and RNA-Seq on embryonic tissues. Performing such studies directly on human tissues is not feasible due to limited availability of human embryos at relevant stages of development. We will therefore in this study exploit the laboratory mouse, a widely used animal model that shares a similar embryonic developmental program and gene regulatory architecture with humans. In addition to embryonic development, our studies will also generate a complementary reference dataset from postnatal and adult mice to better understand the dynamics of gene regulation over time. Furthermore, we will assess the biological authenticity of identified regulatory sequences by in-depth functional validation using an established transgenic mouse pipeline. It is anticipated that generation of these datasets will fill a major void in the functional annotation o a mammalian genome and help to complete the catalog of gene regulatory sequences in the human genome.